Why don't those developing AI Deepfake detectors use two detectors at once so as to catch deepfakes in one or the other?
Because it is possible to fool many different models at once.
See table 2 in this paper, for an example using adversarial perturbations: https://arxiv.org/pdf/1610.08401.pdf
That being said, there is no reason to think that using two detectors at once will not increase chance to detect deepfakes. It will just not resolve the problem completely.